When a stationary light source or an electromagnetic radiation source impinges a planar mirror, the reflection of the light source is projected along a line defined by the location of the mirror relative to the light source, and the orientation of the mirror plane relative to the light source. Placing a screen in the path of the reflected light will show an image of the light source on the screen at a specific location relative to the mirror and source locations. Keeping these four attributes constant (source location, mirror location, mirror plane orientation, screen location) will result in the source image on the screen to remain stationary.
If the location of the light source, the location of the planar mirror, the orientation of the mirror plane or the location of the screen is changed, the image of the light source on the screen will move to a new location on the screen. In order to move the source image back to its original location on the screen, one other attribute or a combination of the other attributes must be changed.
For example, if the light source location is changed, the image on the screen will move to a new location. To move the image back to the original location on the screen, the mirror plane orientation might be changed while keeping the mirror location and screen location constant. Alternatively, the screen location might be changed while keeping the mirror plane orientation and mirror location constant. Finally, a last solution would be to move the mirror location while keeping the screen location and the mirror plane orientation constant.
Focusing Methods for a Moving Light Source
Current focusing systems for tracking moving light sources involve rotational motion of part or all of the focusing system. In a single axis tracker—for a light source moving in a predictable arc—the focusing system—typically a parabolic or Fresnel lens—must rotate around a single axis parallel to the axis of rotation of the source. The axis can be placed anywhere in the system, as long as it is parallel to the source rotational axis. In most applications it is desirable to keep the focus point stationary in space since the energy absorbing medium is located there with its relatively complex and potentially massive interfacing elements such as wiring, conduits (fluid pipes), turbines, or other power generating mechanical items. The focus is thus made to reside on the rotational axis which means the mirror must traverse a circular arc as it tracks the moving light source, and the overall three-dimensional space utilized around the mirror is significant.
In discrete planar array systems, the focus point is also stationary by design and a plurality of individual planar or curved mirrors are kept stationary in a location on the ground and close to the ground, eliminating the need to rotate around the focus. In large mirror array systems there may be thousands of planar or curved mirrors. However, each mirror must be individually and independently rotated and tilted to track the moving light source. While the discrete planar mirror system can reside close to the ground, they still have complex 3-dimensional movement associated with the individual mirrors in the system as both azimuth and altitude angles of the mirror must be adjusted along two axes as the light source moves.
2-Axis Tracking Systems for Planar Arrays
The present inventor has recognized drawbacks with existing 2-axis solar concentrating systems. Much of the existing art utilizes 2-axis tracking in planar array tracking systems. 2-axis tracking requires many components and attributes to be effective. Generally, these 2-axis systems provide a hinge, pivot or gimbal to allow the mirror to rotate on one or two axes, one for vertical (altitude) and one for horizontal (azimuth) rotation. This requires the mirror to be suspended at a fulcrum at the end of a lever arm, which creates a mechanical oscillator system with an inherent mechanical instability. Each mirror sweeps an operational “keep out” area in the shape of a sphere, so that the installation suffers the classic space inefficiency of packing of spheres relative to packing of rectangular prisms. Second, a linkage mechanism is required to actuate changes in rotation on both axes, and the linkages must be affixed to the mirror mechanically and without interference with other elements of the mechanical system. Third, motors with geared shafts must be implemented, one for each axis, to drive the linkage actuators. The installation of linkages and motors places a lower limit on how small the mirror system can be, since space must be reserved for these elements.
Limitations of Two-Axis Systems
The present inventor has recognized limitations associated with existing 2-axis solar concentrating systems. A typical 2-axis tracking system for a planar mirror array will have a relatively large overall component count due to the multiplicative nature of the design, and as such creates challenges to scale the system to very large apertures (total surface area of sunlight captured). The component count for a single mirror system is multiplied by the number of mirrors in the complete system, so as the effective aperture area increases for a set individual mirror size, the total system component count scales in a square law relationship. For example, to double the aperture and thus the power collected, one must quadruple the number of mirrors and so the overall system part count. This translates to a geometric increase in material cost as the aperture is increased. The relatively large component count in a scaled-up system also translates to increased joint count and limitations in overall mechanical reliability such that the system will bear ongoing maintenance cost when in use. Also, a higher part count generally translates to a higher manufacturing time and cost, so the system cost would benefit from the lowest part count possible. Another drawback to a suspended mirror in a 2-axis heliostat design is that in very large mirror applications, the mirror will actually deform (sag) under its own weight around the fulcrum point of suspension.
Several patents have attempted to solve some of the problems associated with 2-axis systems described above.
U.S. Pat. No. 3,466,119 (Francia) discloses a system which attempts to simplify tracking by using a triangulation design that places the mirror into correct initial orientation using telescopic sighting on the desired focus at calibration. While this appears to ultimately simplify the planar array tracking process by reducing tracking adjustment to a single axis, its high part count and associated number of moving joints along with the intricacy of the mirror carrier assembly and manual telescope-enabled calibration suggest a high manufacturing cost associated with the product. An adjustment method for compensating for annual Sun declination angle variation is included in the design, but again involves several moving parts and associated many fulcrum points, adding further to product cost.
U.S. Pat. No. 4,172,443 (Sommer) discloses a mathematical relationship that suggests a means for simplifying the tilt and rotation control of individual stationary mirrors in a planar array by exploiting a natural symmetry in mirror tilt angle components as governed by laws of optical reflection. Although this patent uses a mathematical relationship to allow grouped analog control of a mirror subset, each mirror still requires individual mechanical control of its tilt and rotation components to track the Sun, thus retaining the 2-axis tracking method and therefore remains complex when reduced to practice.
U.S. Pat. No. 5,862,799 (Yogev) discloses a system for controlling individual mirrors in a stationary heliostat mirror field. The fundamental method disclosed is active optical sensing, i.e. while the field is in operation. The imaging aperture reduces the efficiency of the concentrator since part of the focus is consumed by the aperture. Two-axis operation is retained.
U.S. Pat. No. 5,787,878 (Radiff discloses a system which moves a complex of mirrors around a central focus axis and adjusts mirror tilt angles continuously to track the Sun. The system of the '878 patent retains the complex 2-axis design described above, with many mechanical linkages and moving parts to alter both mirror rotation and tilt throughout the tracking period. In addition, this system organizes planar mirrors into separate groups of concave geometries which require complex calibration during manufacturing. The system's use of a hood to capture stray Sun rays accounts for rays inaccurately reflected from the concentrator system. Lastly, the system is not aperture efficient, as large areas of useful sunlight can be left uncaptured throughout the tracking period, and only a centrally located focus can be utilized.
U.S. Pat. No. 4,765,726 (Johnson) and U.S. Pat. No. 6,923,174 (Kurz) disclose transmissive tracker systems which dynamically translate a thin film containing a lens pattern across an aperture to establish light concentration onto a focus. The systems may use either ancillary lenses (Johnson), cylindrical shape (Johnson), or tilt adjustment for operation. These systems however are not easily scalable to very large apertures without introduction of a very large, heavy and expensive lens, or large support elements to maintain the shape of the film which limits its aperture efficiency (i.e. the overall structure is significantly larger than the aperture of the system). Further, the transmissive nature of these concentrators disclose that the aperture be suspended above the focal point, creating limitations in the form of increased wind resistance and space utilization. Longevity requirements of the transparent film material with continuous exposure to the elements, such as ultraviolet light, temperature cycling, and moisture or precipitation, would increase cost in terms of either ongoing maintenance, use of a proprietary robust material, or as Johnson suggests, ancillary protective structure which adds cost and reduces performance.
U.S. Pat. No. 6,959,993 (Gross) and U.S. Pat. No. 7,192,146 (Gross) disclose devices that incrementally and serially adjust the orientation of single mirrors in a heliostat array using a traveling motorized crank that mates with individual turnbuckles in mirror mounts in the array. While this patent describes a new method for translating a control motor within an array, the system uses conventional methods by adjusting mirror tilt and rotation.
The present inventor recognizes that it would be desirable provide a solar concentrator system and method having reduced complexity. The present inventor recognizes that it would desirable to provide a system where the mirrors may have a fixed angle orientation.